Low temperature processing can generally be described as a dehydration, a chemical reaction, a biological reaction, etc. which occurs in a vessel or chamber at temperatures ranging from about -80.degree. C. to about 0.degree. C. The low temperature processes of particular interest in the present invention occur in a vessel or chamber which requires sterilization (or a high temperature process) at the end of the low temperature processing. Examples of such chambers include, but are not limited to, a vacuum chamber for freeze drying and a chemical or biological reactor.
Freeze drying can generally be described as a process of dehydration or of separating water from a product containing biological matter or chemical matter. The product is frozen and then subjected to a high vacuum. The water vaporizes without melting (sublimes) leaving behind non-water components.
Generally, a freeze drying system requires at least four components: a vacuum chamber, a condenser, a vacuum pump, and a means for providing the heat of sublimation to the product being dried or a means for heat transfer. This heat transfer means may comprise a means for heating and a means for cooling. The vacuum chamber typically contains a series of thin stainless steel shelves. Product, for example in containers, is placed upon these shelves. The condenser is used to remove the sublimed water vapor. The pump is typically a high capacity vacuum pump. The freeze drying system typically comprises a sterilization process, especially for pharmaceutical applications.
Freeze drying systems operate over a range of temperatures, but in general the product is completely frozen prior to dehydration. The freezing point of the product may be well below the freezing point of water. For example, the freezing point of the product may be as low as about -50.degree. C. or the operating temperature may be as low as about -50.degree. C.
If sterilization is desired, the freeze drying system may also operate at temperatures around about 121.degree. C. to about 130.degree. C. at atmospheric pressure (i.e., the temperature for high-pressure saturated steam which is often used for sterilization).
During dehydration, a heat-transfer fluid is pumped through passages in the shelves of the vacuum chamber providing the heat of sublimation to the product being dried. Following drying, the product is removed from the container and the vacuum chamber may then be sterilized. As discussed, typically a high temperature steam (121.degree. C. to 130.degree. C. saturation temperature) is used for this sterilization process. If the heat-transfer fluid in the passages boils during this sterilization process, the system pressure may rise to a level where the shelves (which are typically thin to ensure adequate heat transfer) are damaged. Thus, selection of heat-transfer fluid is critical.
Heat-transfer fluids used in such applications typically are liquids having low viscosities at lower temperatures (i.e., -50.degree. C. for the shelves and -80.degree. C. for the condenser system), but are readily maintained in the liquid phase at the highest operating temperature for the system (which is typically during sterilization). Desirable heat-transfer fluids for freeze drying applications are also non-corrosive, non-toxic, and non-flammable.
Polydimethylsiloxanes (silicone oils) have a suitably wide liquid range and are often used in freeze drying. The average molecular weight of the silicone oil can be selected such that it functions well at temperatures as low as -80.degree. C. At this temperature, the heat-transfer fluid may be pumped through passages in the shelves or condenser. Such a silicone oil has a boiling point significantly above 130.degree. C. at atmospheric pressure, thus the passages may be kept full of heat-transfer fluid during the sterilization process without the danger of the heat-transfer fluid boiling and causing elevated system pressure and consequent shelf rupture. Silicone oils seem to be ideally suited for this type of application. However, they are flammable. There have been instances of fires caused by silicone oil and such fires can cost millions of dollars in damages as well as result in injury.
As is the case with freeze drying, during low temperature chemical or biological processes, the heat-transfer fluid preferably has good low temperature heat transfer characteristics. Typically, the heat-transfer fluid is pumped through a reactor jacket for heating, cooling, or temperature control. For ease of handling and safety, preferably this fluid is non-toxic and non-flammable. The heat-transfer fluid has similar temperature constraints (i.e., suitable at low temperature processing temperatures and at high temperature sterilization temperatures).
Fluorinated organic compounds, such as perfluorocarbons (PFCs), perfluoropolyethers (PFPEs), hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrohalofluoroethers (HHFEs), or mixtures thereof, are generally non-toxic and non-flammable. The lower molecular weight compounds tend to have good low temperature heat transfer properties. Additionally, fluorinated organic compounds are non-corrosive and thermally stable.
However, some of these fluorinated organic compounds, such as HFCs, HCFCs, and HHFEs, tend not to be viable candidates as heat-transfer fluids because either their boiling points are too low (leading to excessive heat-transfer system pressure at high temperatures) or their freezing points are too high (leading to freeze-up or high viscosity at low temperatures). Candidates, for example, that are liquid at 130.degree. C. or which have acceptable vapor pressures at this temperature tend to be solid or very viscous at -80.degree. C. and thus cannot be used. Similarly, candidates, for example, that may work well at -80.degree. C. tend to have lower boiling points which result in excessive vapor pressures that would prevent their use at 130.degree. C. Typically, these fluids are not used in conventional designs because, to maintain the fluid in a liquid state throughout the system/apparatus and throughout the operating temperature range, the heat-transfer system is typically pressurized above the fluid saturation pressure using a compressed gas such as air or nitrogen. This pressure compromises the integrity of certain components in the apparatus unless they are built to more rigorous design codes which adds cost and may affect performance.
Other fluorinated organic compounds, such as PFCs and PFPEs, have long atmospheric lifetimes and/or high global warming potentials. CFCs may no longer be produced commercially due to ozone depletion concerns.
Thus, the need exists for a heat-transfer fluid for low temperature processes requiring high temperature sterilization which has good heat transfer properties at low temperatures, non-corrosivity, non-flammability, low toxicity, good environmental properties, and does not boil at sterilization temperatures.